RFC 6854

Update to Internet Message Format to Allow Group Syntax in the "From:" and "Sender:" Header Fields

Internet Engineering Task Force (IETF) B. Leiba
Request for Comments: 6854 Huawei Technologies
Updates: 5322 March 2013
Category: Standards Track
ISSN: 2070-1721
Update to Internet Message Format to Allow Group Syntax in
the "From:" and "Sender:" Header Fields
Abstract
The Internet Message Format (RFC 5322) allows "group" syntax in some
email header fields, such as "To:" and "CC:", but not in "From:" or
"Sender:". This document updates RFC 5322 to relax that restriction,
allowing group syntax in those latter fields, as well as in
"Resent-From:" and "Resent-Sender:", in certain situations.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6854.
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This document therefore updates the Internet Message Format
specification [RFC5322] to relax that restriction, allowing group
syntax to be used in the originator ("From:" and "Sender:") fields,
as well as in their corresponding resent ("Resent-From:" and
"Resent-Sender:") fields. This change permits empty groups, as
described above, and also permits named groups of mailboxes (groups
with non-empty lists of addresses; see Section 4). Nevertheless,
this document recommends against the general use of group syntax in
these fields at this time (see Section 3).
1.1. Notational Conventions
The notational conventions here are the same as those in RFC 5322,
and the following two subsections are copied directly from that
document.
1.1.1. Requirements Notation
This document occasionally uses terms that appear in capital letters.
When the terms "MUST", "SHOULD", "RECOMMENDED", "MUST NOT", "SHOULD
NOT", and "MAY" appear capitalized, they are being used to indicate
particular requirements of this specification. A discussion of the
meanings of these terms appears in the Key Words document [RFC2119].
1.1.2. Syntactic Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
[RFC5234] notation for the formal definitions of the syntax of
messages. Characters will be specified either by a decimal value
(e.g., the value %d65 for uppercase A and %d97 for lowercase A) or by
a case-insensitive literal value enclosed in quotation marks (e.g.,
"A" for either uppercase or lowercase A).
2. Allowing Group Syntax in "From:" and "Sender:"
Section 3.6.2 of RFC 5322 defines the "From:" header field as
containing a <mailbox-list> syntax element. This specification
changes that definition to use the <address-list> syntax element, as
is used in other fields, such as "To:", "CC:", and "Reply-To:". This
specification also changes the definition of the "Sender:" header
field from the <mailbox> syntax element to the <address> syntax
element. While the <address> element includes the <mailbox> element
already, we have chosen to specify both in the updated syntax as a
way of highlighting the limited use intended for the change (see
Section 3).

Section 2.1 below is a full replacement for Section 3.6.2 of RFC
5322, containing the new syntax as well as a new description of the
semantics for the "From:" and "Sender:" fields. Section 2.2 below is
a replacement of only the ABNF syntax for the "Resent-From:" and
"Resent-Sender:" fields in Section 3.6.6 of RFC 5322; the rest of the
syntax as well as the descriptive text of Section 3.6.6 of RFC 5322
remains unchanged.
[The text in the following section is not consistent within itself
nor with the rest of this document in how it refers to message header
fields, sometimes putting the field name in quotation marks and
sometimes not, sometimes capitalizing the field name and sometimes
not, and sometimes including the final colon and sometimes not.
Because minimizing changes to the original text is more important, in
this case, than attaining consistency, the text in Section 2.1, as
well as that in Sections 1.1.1 and 1.1.2 above, is left as it was in
RFC 5322.]
2.1. Replacement of RFC 5322, Section 3.6.2. Originator Fields
The originator fields of a message consist of the from field, the
sender field (when applicable), and optionally the reply-to field.
The from field consists of the field name "From" and a
comma-separated list of one or more addresses (either mailbox or
group syntax). If the from field contains more than one mailbox
specification (including all mailboxes included in any groups), then
the sender field, containing the field name "Sender" and a single
address, MUST appear in the message. The from field and the sender
field SHOULD NOT use group syntax; rather, the from field SHOULD use
only the mailbox-list syntax and the sender field SHOULD use only
mailbox syntax (see RFC 6854, Section 3). If the sender field uses
group syntax, the group MUST NOT contain more than one mailbox. In
either case, an optional reply-to field MAY also be included, which
contains the field name "Reply-To" and a comma-separated list of one
or more addresses.
from = "From:" (mailbox-list / address-list) CRLF
sender = "Sender:" (mailbox / address) CRLF
reply-to = "Reply-To:" address-list CRLF
The originator fields indicate the mailbox(es) of the source of the
message. The "From:" field specifies the author(s) of the message,
that is, the mailbox(es) of the person(s) or system(s) responsible
for the writing of the message. The "Sender:" field specifies the
mailbox of the agent responsible for the actual transmission of the
message. For example, if a secretary were to send a message for

another person, the mailbox of the secretary would appear in the
"Sender:" field and the mailbox of the actual author would appear in
the "From:" field. If the originator of the message can be indicated
by a single mailbox and the author and transmitter are identical, the
"Sender:" field SHOULD NOT be used. Otherwise, both fields SHOULD
appear.
Note: The transmitter information is always present. The absence
of the "Sender:" field is sometimes mistakenly taken to mean that
the agent responsible for transmission of the message has not been
specified. This absence merely means that the transmitter is
identical to the author and is therefore not redundantly placed
into the "Sender:" field.
The originator fields also provide the information required when
replying to a message. When the "Reply-To:" field is present, it
indicates the address(es) to which the author of the message suggests
that replies be sent. In the absence of the "Reply-To:" field,
replies SHOULD by default be sent to the mailbox(es) specified in the
"From:" field unless otherwise specified by the person composing the
reply.
In all cases, the "From:" field SHOULD NOT contain any mailbox that
does not belong to the author(s) of the message. See also Section
3.6.3 of RFC 5322 [RFC5322] for more information on forming the
destination addresses for a reply.
2.2. Update to RFC 5322, Section 3.6.6. Resent Fields
This section updates RFC 5322, Section 3.6.6, to allow groups (via
the address-list ABNF production) in the "Resent-From:" and
"Resent-Sender:" fields, to parallel the change to "From:" and
"Sender:" above. The ABNF for these fields is changed as follows:
resent-from = "Resent-From:" (mailbox-list / address-list) CRLF
resent-sender = "Resent-Sender:" (mailbox / address) CRLF

3. Applicability Statement
Mailbox syntax is the normal syntax to use in the "From:" and
"Sender:" header fields; the address syntax defined in Section 2.1,
which allows the specification of a group, is only for Limited Use
(see RFC 2026 [RFC2026], Section 3.3, item (d)) for the reasons
described below.
Many Internet email procedures and much software assumes that the
addresses in the "From:" and "Sender:" fields can be replied to and
are suitable for use in organizing and filtering mail. The use of
groups instead of mailboxes can disrupt these uses. Consequently,
while this specification legitimizes the use of groups, it does so
only to enable circumstances when that use is necessary. Because the
use of this mechanism is new, it is important that its use be limited
to these circumstances and that it be used with caution. In
particular, user agents SHOULD NOT permit the use of groups in those
fields in outgoing messages.
4. Examples
First, consider an email message that is sent by an automated nightly
monitor program, to which replies should not be sent. Such messages
commonly include a valid, replyable address that will discard any
replies that are sent to it, but recipients who do reply might be
unaware that their replies will be discarded. If the message is
instead presented as follows, the recipients' email clients will not
allow them to reply in the first place:
From: Nightly Monitor Robot:;
Second, consider an email message that is meant to be "from" the two
managing partners of a business, Ben and Carol, and that is sent by
their assistant, Dave. This message could always have been presented
this way:
From: ben@example.com,carol@example.com
Sender: dave@example.com
This change allows it to be represented this way:
From: Managing Partners:ben@example.com,carol@example.com;
Sender: dave@example.com

5. Security Considerations
See the Internet Message Format specification [RFC5322] for general
discussion of security considerations related to the formatting of
email messages.
The "From:" address is special, in that most user agents display this
address, or the "friendly" text associated with it, to the end user,
and label it so as to identify it as the origin of the message (as
implied in Section 3.6.2 of RFC 5322). Group syntax in the "From:"
header field can be used to hide the identity of the message
originator. It is just as easy to use a fabricated "From:" address
to accomplish the same thing, so allowing groups in this field does
not exacerbate the security problem.
Some protocols attempt to validate the originator address by matching
the "From:" address to a particular verified domain (for one such
protocol, see the Author Domain Signing Practices (ADSP) document
[RFC5617]). Such protocols will not be applicable to messages that
lack an actual email address (whether real or fake) in the "From:"
field. Local policy will determine how such messages are handled,
and senders, therefore, need to be aware that using groups in the
"From:" might adversely affect deliverability of the message.
Because groups have previously not been allowed in the "From:" and
"Sender:" header fields, it is possible that some implementations
that conform to RFC 5322 might not be prepared to handle the group
syntax, and, indeed, might not even recognize that group syntax is
being used. Of those implementations, some subset might, when
presented with group syntax in those header fields, behave in a way
that is exploitable by an attacker. It is deemed unlikely that this
will be a serious problem in practice: address field parsing is
generally an integral component of implementations, and address field
parsers are required to understand group syntax. In addition, if any
implementations should be exploitable through this mechanism, it is
already possible for attackers to do it by violating RFC 5322. Other
violations of RFC 5322 are commonly used by malefactors.